How to install Blender while retaining addons
New Blender versions can be installed along side the previous versions, meaning that your old install of Blender will continue to work with its add-ons.

For the new version of Blender you will have to install the add-ons again. If the add-on you've previously downloaded / purchased is compatible with the new version of Blender, you can use the same files (.zip, .py) for the installation of the add-on. Should they not be compatible, then you will have to check if an updated version of the add-on is available for that specific Blender version. 

Add-ons for 2.79b and earlier can't be used in Blender 2.8x because of the changed API.